1. Field of the Invention
The present invention relates to a dual injection type internal combustion engine and, more specifically, to a dual injection type internal combustion engine including an injector for in-cylinder injection for injecting fuel to the inside of a cylinder and an injector for intake manifold injection for injecting fuel to the intake manifold or to an intake port, provided with learning means for learning background noise level used as a reference at the time of knock determination.
2. Description of the Background Art
A so-called dual injection type internal combustion engine has been known that generally includes an injector for in-cylinder injection for injecting fuel to the inside of a cylinder and an injector for intake manifold injection for injecting fuel to the intake manifold or to an intake port, in which the injectors are switched and used in accordance with the operating state of the engine, to realize, for example, stratified charge combustion in a low-load operating range and homogeneous combustion in a high-load operating range or to inject fuel from each of the injectors at a prescribed contribution ratio in accordance with the operating state, so as to improve mileage and output characteristics.
As is well known, in many internal combustion engines, a knock determination is made to determine the presence or absence of knock and knock control is performed in which spark timing and the like is adjusted in accordance with the determination results. Ordinarily, the knock determination is made using a knock sensor, which is a vibration detecting sensor mounted on the cylinder block or the like. The presence or absence of knock is then detected based on an output signal from the knock sensor during a predetermined period (knock determination period or gate period) set close to the compression top dead center of each cylinder (see, for example, Japanese Patent Laying-Open No. 2004-251218).
The output signal of the knock sensor includes, in addition to the signals derived from knocking, various signals that come from the vibration of engine itself. Such signals are referred to as a background noise, a threshold value is set using the background noise level as a reference, and when the output value of the knock sensor exceeds the threshold value in the knock determination, it is determined that knock is occurring.
The background noise level varies in accordance with the operating state of the engine. Generally, the higher the engine speed and the higher the engine load, the higher becomes the background noise level.
As the background noise level varies in accordance with the operating state of the engine, the value of the background noise level set in a prescribed operating range of the engine is not always a suitable value in another operating range of the engine. By way of example, when a threshold value is set using a low background noise level as a reference in a low speed range of the engine and the engine enters a high speed operating state, the background noise level would increase and the sensor output value exceeds the threshold value because of the background noise itself, resulting in an erroneous determination that knock is occurring.
Therefore, in such a case, it is necessary to newly set a high background noise level for a high-speed operating state of the engine, and to newly set a high threshold value for the high-speed range. Learning of a background noise level based on an actual output signal of the knock sensor in a certain operating range of the engine is “background learning.” When the background learning is done in the high-speed range, it becomes possible to set a high background noise level for the high-speed range and to set a high threshold value.
When background learning is being executed, however, the output signals from the knock sensor are treated exclusively as the background noise while the threshold value for knock determination is not yet determined, and therefore, knock determination is impossible. Accordingly, it is also impossible to perform knock control for adjusting the spark timing and the like in accordance with the knock determination results and hence, knocking may possibly occur.
As disclosed in Japanese Patent Laying-Open No. 2004-251218, in an in-cylinder injection type internal combustion engine, operational noise of the injector at the start and end timings of injection of the in-cylinder injector may ride on the output signals of the knock sensor, forming a part of the background noise. During background learning, such a state having the operational noise of the injector riding on the signals is rather preferable, as it enables accurate background learning.